A state of the art microprocessor architecture may have one or more caches for storing data and instructions local to the microprocessor. A cache may be disposed on the processor chip itself or may reside external to the processor chip and be connected to the microprocessor by a local bus permitting exchange of address, control, and data information. By storing frequently accessed instructions and data in a cache, a microprocessor has faster access to these instructions and data, resulting in faster throughput.
Conventional microprocessor-cache architectures were developed for use in computer systems having a single computer processor. Consequently, conventional microprocessor-cache architectures are inflexible in multiprocessor systems in that they do not contain circuitry or system interfaces which would enable easy integration into a multiprocessor system while ensuring cache coherency.
A popular multiprocessor computer architecture consists of a plurality of processors sharing a common memory, with each processor having its own local cache. In such a multiprocessor system, a cache coherency protocol is required to assure the accuracy of data among the local caches of the respective processors and main memory. For example, if two processors are currently storing the same data block in their respective caches, then writing to that data block by one processor may effect the validity of that data block stored in the cache of the other processor, as well as the block stored in main memory. One possible protocol for solving this problem would be for the system to immediately update all copies of that block in cache, as well as the main memory, upon writing to one block. Another possible protocol would be to detect where all the other cache copies of a block are stored and mark them invalid upon writing to one of the corresponding data block stored in the cache of a particular processor. Which protocol a designer actually uses has implications relating to the efficiency of the multiprocessor system as well as the complexity of logic needed to implement the multiprocessor system. The first protocol requires significant bus bandwidth to update the data of all the caches, but the memory would always be current. The second protocol would require less bus bandwidth since only a single bit is required to invalidated appropriate data blocks. A cache coherency protocol can range from simple, (e.g., write-through protocol), to complex, (e.g., a directory cache protocol). In choosing a cache coherence protocol for a multiprocessor computer system, the system designer must perform the difficult exercise of trading off many factors which effect efficiency, simplicity and speed. Hence, it would be desirable to provide a system designer with a microprocessor-cache architecture having uniquely flexible tools facilitating development of cache coherence protocols in multiprocessor computer systems.
A present day designer who wishes to construct a multiprocessor system using a conventional microprocessor as a component must deal with the inflexibility of current microprocessor technology. Present day microprocessors were built with specific cache protocols in mind and provide minimal flexibility to the external system designer. For example, one common problem is that a cache of a microprocessor is designed so that a movement of a data block out of a cache automatically sets the cache state for the block to a predetermined state. This does not give a designer of a multiprocessor system the flexibility to set the cache to any state in order to implement a desired cache protocol. Because of this significant complexity is necessarily added to the design of a cache protocol.